This invention relates to a photo-mask blank for use in manufacturing a semiconductor device, an integrated circuit, a large-scale integrated circuit, and the like and to a photo-mask made from the photo-mask blank.
A conventional photo-mask blank of the type described comprises a transparent substrate member having a principal surface and a shading layer of chromium (hereafter frequently referred to as a chromium layer) coated on the principal surface. The transparent substrate member may comprise a transparent plate and a transparent conductive layer attached to the transparent plate. The transparent plate may be of soda-lime glass, borosilicate glass, silica glass, or sapphire while the transparent conductive layer is made of indium oxide, tin oxide, or the like. The shading or chromium layer is frequently covered with an antireflection layer. The chromium layer and the chromium oxide layer are formed by sputtering, ion plating, or the like.
At any rate, the chromium layer is selectively etched by an etchant with a photoresist covered on the photo-mask blank, so as to leave a predetermined pattern of the chromium on the principal surface.
It should be noted here that objectionable dust is inevitably attached to the photoresist when the photoresist is made to adhere to the photo-mask blank by coating and baking the photoresist on the photo-mask blank. Such objectionable dust can be eliminated together with the photoresist on removal of the photoresist. However, the dust unpleasingly gives rise to undesired spots of chromium on the substrate member after exposure of the photoresist. This is because the photoresist is not exposed at the portions to which the dust is attached. The spots become serious when a fine pattern is delineated as the predetermined pattern.
The spots may be removed by carrying out excessive etching or overetching because they are very small. However, the excessive etching adversely affects the predetermined pattern and makes it difficult to delineate a precise pattern.